EP0663562A2 - Process and burner for reducing harmful gas emissions during combustion - Google Patents

Abstract

Harmful gas emissions from combustion processes are reduced by oxidising the fuel with an oxygen contg. oxidising agent in the presence of recirculating waste gases. The oxidising agent is split up into a main oxygen stream (23) and a primary oxygen stream (14). Before the main oxygen stream (23) take part in the oxidation of the fuel, it is preheated to at least 100 degrees C and the flow is pulsed in relation to the combustion capacity so that at least 23.6 N/MW is generated with stoichiometric combustion. The fuel is fed in through a water cooled supply channel (16) in a pulsed flow related to the combustion capacity. The N/MW generated at the outlet is between 2 and 18% of the burner's pulsed flow, depending on the type of fuel. The primary oxygen stream (14) is fed in a temp. plus or minus 30 degrees C. 0.4% of the stoichiometric quantity of oxygen required and the primary oxygen flow with the emergent part-oxidised combustible material is finally mixed with the main oxygen flow and the recirculating combustion waste gases. Also claimed is a burner designed for the combustion process described above.

Description

The invention relates to a method for reducing harmful gas emissions during combustion according to the preamble of claim 1 and a burner for carrying out the method according to the preamble of claim 11.

In the past, combustion processes that work with oxygen as an oxidizing agent have always been used when increasing the performance of a furnace system, saving primary energy or reducing the volume of combustion gases. In addition to these main advantages of an oxygen application, the ignition behavior and flame stability have been improved and higher process temperatures can be achieved.

In the past, the combustion processes were primarily optimized with regard to burnout by completely burning off the resulting CO gases and CH compounds.

The 800 to 1000 ° C higher adiabatic flame temperatures when using technically pure oxygen compared to fuel / air combustion lead to strong nitrogen oxide (NOx) formation.

The nitrogen oxides generated as by-products during combustion are considered to be the cause of acid rain and the increased ozone formation (damage to forests and people).

Although the dependencies and qualitative proportions of the NOx formation mechanisms have not yet been fully clarified, it is now considered certain that the flame temperature> 1300 ° C, the residence time of oxygen at high temperatures and the oxygen partial pressure are the main influencing factors of thermal NOx formation.

At the same time, NOx is formed on the basis of the nitrogen that is organically bound in the fuel. Promt NO is also formed by free oxygen reacting during the combustion process.

The main requirements placed on low-pollutant combustion, such as stable ignition behavior, enabling large control ranges without flame instabilities and ensuring complete burnout, have significantly influenced the combustion concepts and the aspects of the necessary burner designs.

The previously known low-pollutant combustion processes and the associated combustion devices involve NOx reduction measures for near-stoichiometric combustion, avoidance of flame temperature peaks through recirculation of combustion exhaust gases into the combustion zone and multi-stage combustion by dividing the required amount of oxygen into at least two partial flows.

Scientific studies over the past decade have confirmed that the flame temperature peaks are the main cause of thermal NOx formation. This resulted in the requirement to create effective flame cooling. The flames can be cooled by recirculation of combustion gases and mixing into the combustion zone or by internals in the combustion zone. The internals are exposed to strong heat, so that they are subject to high wear.

In a known combustion concept for low-emission combustion with oxygen, an impulsive flow of the main oxygen flow was provided for the recirculation of the combustion exhaust gases.

Laval nozzles, in which the pressure energy is converted into speed, have been proposed for generating the pulsed flow of the main oxygen stream (US Pat. No. 5,104,310).

The disadvantages are the high manufacturing costs of these burners and the insufficient control range of the burner. Low pollutant gas emissions are achieved primarily for the design state, since the Laval nozzles only work optimally in a narrow area.

In the known combustion processes for low-emission combustion, at least 5 to 10% of the oxygen required for stoichiometric combustion is required for flame stabilization. Due to these large amounts of oxygen, NOx is additionally formed in the area of the flame root.

The use of burner stones for flame stability while simultaneously reducing the primary oxygen quantities to 1 to 5% leads to additional costs, since the burner stones are made of high-quality materials and are exposed to premature wear due to the high thermal loads.

The previously known methods for low-emission combustion only partially meet the demands made with regard to reduced NOx emissions with simultaneous economical use of the fuel (no unburned fuel). They only work economically in a limited area.

The invention has for its object to provide a method and a device that allow an oxidation of fuel with a predominantly oxygen-containing oxidizing agent under economic conditions with simultaneously lower pollutant formation (NOx) over a large control range.

According to the invention, this object is achieved in a generic method by the characterizing features of claim 1 and in a burner for carrying out the method by the characterizing features of claim 11.

Advantageous developments of the invention are specified in the subclaims.

The achievement of the greatest possible additional gain in an impulsive flow of the main oxygen flow, which is necessary for the recirculation of the combustion exhaust gases, is achieved according to the invention in that the oxygen-containing oxidizing agent with a minimum oxygen content of 90% to a temperature greater than 100 ° C by the combustion exhaust gases or by an external Device is preheated. This enables a pulsed flow of at least 23.6 N / MW related to the combustion output to be generated. The pulsed flow of the main oxygen flow required for the mixing in of the combustion gases is almost compensated for by changes in the burner's output (reduction in the amount of the main oxygen flow) by increasing the preheating temperature in the control range of 1: 8.

This ensures the pulsed flow of the main oxygen flow even when the burner changes load, so that the NOx and CO emissions remain constantly low regardless of the load range.

The design of the nozzles for the main oxygen flow with an outlet cross-section of at least 350 mm² / MW related to the combustion output ensures that the flow conditions adapt to the changing burner output and thus decisively enable the economical use of the fuel under environmentally satisfactory conditions, with a stable flame signal for safety-related monitoring of the burner over the entire control range is.

The inventive arrangement of the main oxygen nozzles has created favorable conditions for the outer diameter of the burner to be small.

The arrangement and dimensions of the supply duct for the primary oxygen flow in the water-cooled supply duct for the fuel and the predetermined primary oxygen temperature of ≦ 30 ° C ensure that a stable pilot flame with small amounts of primary oxygen is guaranteed, which provides a sufficient UV signal to monitor the Brenners delivers. This avoids temperature peaks in the flame root due to the low primary oxygen flow with far below stoichiometric combustion. There is no need to use burner stones.

Avoid the inventive design of the water-cooled supply channel for fuel and the fuel pulse flow related to the combustion output in connection with the temperature of the primary oxygen flow of ≦ 30 ° C and the low proportion of the primary oxygen flow of <1% of the stoichiometrically required amount of oxygen, temperature peaks in the flame root and thus Pollutant emissions. The flame is kept stable at the outlet and burns at a low flame temperature.

The method according to the invention and the burners according to the invention reduce the harmful gas emissions and are particularly suitable for heating industrial furnaces due to stable and complete combustion. The tightened Limit values for nitrogen oxides and carbon monoxide are well undercut, so that the entire furnace system can be operated economically under environmentally friendly conditions.

The new process can be carried out with gaseous and / or liquid atomized fuel.

The invention will be explained in more detail below using exemplary embodiments. In the following, exemplary embodiments of the burners according to the invention are shown schematically in the drawings.

Fig. 1

einen Schnitt durch einen Brenner zur Durchführung des Verfahrens;

Fig. 2

die Ansicht A des Brennstoffaustrittes des Brenners nach Fig. 1;

Fig. 3

einen Schnitt durch einen Brenner zur Durchführung des Verfahren mit indirekt, über die Strahlungswärme des Ofens vorgewärmten Hauptsauerstoffstrom in konzentrisch angeordneten Zuleitungskanälen;

Fig. 4

einen Schnitt durch einen wassergekühlten Brenner mit externer Sauerstoff (O₂)-Vorwärmung für den Hauptsauerstoffstrom;

Fig. 5

ein Diagramm zur Veranschaulichung der auftretenden NOx-Bildung in Abhängigkeit von der Erdgasqualität und der Prozeßtemperatur.

Show it

Fig. 1

a section through a burner for performing the method;

Fig. 2

the view A of the fuel outlet of the burner according to Fig. 1;

Fig. 3

a section through a burner for performing the method with indirectly, via the radiant heat of the furnace preheated main oxygen flow in concentrically arranged supply channels;

Fig. 4

a section through a water-cooled burner with external oxygen (O₂) preheating for the main oxygen stream;

Fig. 5

a diagram to illustrate the occurring NOx formation as a function of the natural gas quality and the process temperature.

In the burners shown in FIGS. 1, 3 and 4, the same parts have been given the same reference numbers. The burner shown in FIG. 1 consists of a central supply duct 14 for the primary oxygen flow which is arranged on the central axis 10 of the water-cooled supply duct 16 for fuel flow and is therefore surrounded concentrically by the supply duct 16. The supply duct 14 is arranged to be displaceable in the supply duct 16 for the fuel flow in accordance with the arrows 21 and preferably closes on one level with the outlet of the supply duct 16 for the fuel flow. The supply channel 16 for the fuel flow is surrounded concentrically by supply channels 17a, 17b for a cooling medium, preferably water. The cooling medium flows via line 18 into the supply channel 17a, is led to the immediate vicinity of the outlet 19 of the fuel flow, is deflected here and flows via line 20 from the supply channel 17b. Outside of the water-cooled supply duct 16 for the fuel flow, supply ducts 13 for main oxygen are arranged. The supply channels 13 are connected via a ring channel 22 and line 23 to a supply source for oxygen, not shown. As FIG. 1 and FIG. 2 show, the supply channels 13 for the main oxygen flow are arranged at a lateral distance from the supply channels 17a, 17b for coolant.

bestimmte Wert ist, wobei d_a in mm gemessen ist, F in mm²/MW und η die Anzahl der Austrittskanäle der Hauptsauerstoffdüsen ist. Der wassergekühlte Zuleitungskanal 16 hat am Austritt 19 des Brennstoffes einen Abstand A zu den Austrittskanälen 11, 12 der Hauptsauerstoffdüsen 24 bis 31 der einen durch die Ungleichung

${\text{A < 3,9 d}}_{\text{a}}$

bestimmten Wert hat, wobei d_a der Durchmesser der Austrittskanäle 11, 12 der Hauptsauerstoffdüsen ist.The eight feed channels 13 shown in FIGS. 1 and 2 consist of separate tubes and, starting from the annular channel 22, run essentially parallel to the central axis 10 up to the vicinity of the outlet 19 of the fuel flow, bend here and each open into a nozzle body 33 arranged eight main oxygen nozzles 24 to 31. At least are advantageous two, preferably at least four main oxygen nozzles are provided on a circle 32. The outlet channels 11, 12 (FIG. 1) of the main oxygen nozzles run parallel to the outlet channel 9 of the water-cooled feed channel 16 for the fuel flow and have a minimum center distance L from one another which is a multiple, preferably at least 3 times, the nozzle diameter d _{a of} the main oxygen nozzles. The nozzles 24 to 31 for the main oxygen stream have an outlet cross section F of at least 350 mm 2 / MW, based on the combustion output, the preferably at least four outlet channels 11, 12 for the main oxygen stream arranged in a circle 32 around the water-cooled supply channel 16 for the fuel stream having a diameter d have _a greater than by the formula

$${\text{d}}_{\text{a}}\text{= 1.13 (}\frac{\text{F}}{\text{η}}{\text{)}}^{\text{½}}$$

is a certain value, where d _{a is} measured in mm, F in mm² / MW and η is the number of outlet channels of the main oxygen nozzles. The water-cooled supply duct 16 is at the outlet 19 of the fuel at a distance A from the outlet ducts 11, 12 of the main oxygen nozzles 24 to 31 due to the inequality

${\text{A <3.9 d}}_{\text{a}}$

has a certain value, where d _{a is} the diameter of the outlet channels 11, 12 of the main oxygen nozzles.

The burner is arranged in a closed combustion chamber 35. A primary oxygen stream flows via line 36 into the feed channel 14 and a fuel stream flows into the water-cooled feed channel 16 via line 37. The outlet channel 9 for the fuel stream is designed according to an embodiment not shown in detail as a nozzle which narrows towards the outlet 19 and constricts the fuel flow. The speed of the primary oxygen flow is between 1.5 and 10 m / s and the gaseous fuel flow between 15 and 75 m / s. The proportion of primary oxygen is at least 0.4% of the stoichiometrically required amount of the oxidizing gas. After ignition, fuel partially burns with the primary oxygen. A stable pilot flame is formed which emits a sufficient signal for a sensor which is preferably designed as a UV light receiver 34 and which is connected to an evaluation unit for monitoring the burner. The primary oxygen stream flows into the feed duct 14 at a temperature <30 ° C. The main oxygen stream is preheated to a temperature of at least 100 ° C. before being oxidized with the fuel. In the case of burners according to the exemplary embodiments in FIGS. 1 and 3, the preheating takes place via the radiant heat of the combustion exhaust gases. For this purpose, the supply ducts 13 for the main oxygen are on the outside and, as shown in the burner according to FIG. 1, are arranged at a lateral distance from the supply ducts 17a, 17b for coolant, so that they are exposed to the radiant heat from all sides. In the burner according to FIG. 3, the main oxygen flows via the line 23 into the supply duct 13a, which is acted upon by the radiant heat of the combustion exhaust gases. The main oxygen flow is deflected via feed channels 13b, 13c running parallel to the central axis 10 before it emerges from the main oxygen nozzles 24 to 31.

In the burner shown in Fig. 4, a device 38 for the main oxygen is via line 23 Preheating connected, by means of which the main oxygen is preheated to at least 100 ° C. 1 and 3, the supply duct 13 for the main oxygen is surrounded by a supply duct 39 for a cooling medium, which flows into the supply duct 39 via line 40, is deflected in the region of the outlet 19 of the fuel and out via line 41 to the burner. This burner can be operated at high furnace temperatures.

The main oxygen stream emerging from the main oxygen nozzles 24 to 31 of FIGS. 1 to 4 of the burner flows with a pulse stream related to the combustion output with stoichiometric combustion of at least 23.6 N / MW and a speed of greater than 310 m / s. It causes an intensive recirculation of the combustion exhaust gases, which mix with the main oxygen flow and the fuel gas and primary oxygen flow. The fuel pulse flow, based on the combustion output, of at least 0.5 N / MW at the outlet 19 is, depending on the type of fuel, between 2% and 30%, preferably between 2% and 18% of the pulse flow of the oxidizing gas. There is a low-pollutant, in particular low-NOx, oxidation due to the reduction of temperature peaks and a uniform temperature field ∸ of the flame. Furthermore, the inventive design of the water-cooled supply channel 16 for fuel and the fuel pulse flow related to the combustion power in combination with the primary oxygen temperature ≦ 30 ° C. and the low proportion of the primary oxygen of <1% of the stoichiometrically required amount of oxygen prevent temperature peaks ∸ of the flame root. The oxygen content of the oxidizing gas is advantageously at least 90%.

The burners described in the exemplary embodiments in connection with FIGS. 1 to 4 for carrying out the method are distinguished in that the nitrogen oxide emissions are drastically reduced. 5 shows the NOx content of dry combustion exhaust gas as a function of the furnace temperature.

The hatched area A illustrates the NOx emissions from conventional natural gas / oxygen burners that work without measures to reduce NOx. In the entire temperature range, the NOx emissions are far above the limit value of 500 mg / m³ of the TA-Luft.

The results of the NOx emissions of the method according to the invention are shown in hatched area B as a function of the natural gas quality and the furnace temperature. As shown in FIG. 5, the burner operating according to the method of the present invention has considerably more favorable conditions, with the TA air limit being significantly undercut.

The reduction rates become even clearer when the method according to the invention is used for the harmful gases NOx and CO if the emissions are related to the enthalpy. The limit values according to TA-Luft are approx. 450 mg / kWh for NOx and approx. 90 mg / kWh for CO. The actual values of the lower curve of area B are between 3.5 and 28 mg / kWh for NOx and between 6.0 and 18 mg / kWh for CO. The result of this is that the process according to the invention produces the lowest harmful gas emissions while at the same time using energy economically.

Claims (17)

Process for reducing harmful gas emissions when combusting fuel in a combustion chamber, in which the fuel is oxidized with a predominantly oxygen-containing oxidizing agent in the presence of recirculated combustion exhaust gases,
characterized in that - The oxidizing agent is divided into a main oxygen stream and a primary oxygen stream; - the main oxygen stream, before it participates in the oxidation of the fuel, preheated to a temperature of at least 100 ° C. and a pulse stream of the main oxygen stream related to the combustion power is generated with stoichiometric combustion of at least 23.6 N / MW; - The fuel in a preferably water-cooled supply channel is fed as a fuel flow to the combustion and a fuel pulse flow in N / MW at the outlet, based on the combustion output, depending on the type of fuel, between 2% and 30%, preferably between 2% and 18%, of the burner Pulse current is generated; - The primary oxygen stream is supplied at a temperature ≦ 30 ° C, the proportion <1%, preferably 0.4%, of the stoichiometrically required amount of oxygen and the primary oxygen stream is partially oxidized with the escaping fuel and then mixed with the main oxygen stream and recirculated combustion exhaust gases. Method according to claim 1,
characterized in that
the main oxygen flow from the fuel flow one Distance A, which results from the outer edge of the fuel flow to the outer edge of the main oxygen flow at the outlet and that due to the inequality

${\text{A <3.9 d}}_{\text{a}}$

has a certain value, where d _{a is} the diameter of the main oxygen stream at the outlet. The method of claim 1 or 2,
characterized in that
the main oxygen flow has an exit cross-section based on the combustion capacity of at least 350 mm² / MW. Method according to one of claims 1 to 3,
characterized,
that the oxygen content in the oxidizing agent is at least 90%. Method according to one of claims 1 to 4,
characterized,
that the main oxygen stream is preheated by means of the radiation energy of the combustion exhaust gases. Method according to one of claims 1 to 4,
characterized,
that the main oxygen stream is preheated by an external preheater. Method according to one of claims 1 to 6,
characterized,
that the speed of the main oxygen flow is greater than 310 m / s. Method according to one of claims 1 to 7,
characterized,
that the main oxygen stream from at least 2, preferably at least 4, arranged in a circle around the water-cooled supply duct for fuel is supplied to the combustion. Method according to one of claims 1 to 8,
characterized,
that the speed of the fuel flow is between 15 and 75 m / s. Method according to one of claims 1 to 9,
characterized,
that the speed of the primary oxygen flow is between 1.5 and 10 m / s. Burner for performing the method according to one of the preceding claims 1 to 10, in which the fuel is oxidized with a predominantly oxygen-containing oxidizing agent in the presence of recirculated combustion exhaust gases in a combustion chamber, with nozzles for a main oxygen stream for recirculation of the combustion exhaust gases and a primary oxygen stream,
characterized in that - The nozzles (24 to 31) for the main oxygen stream generate a pulse stream related to the combustion power of at least 23.6 N / MW; - The nozzles (24 to 31) for the main oxygen flow have an outlet cross section F of at least 350 mm² / MW based on the combustion output, at least 2, preferably at least 4 nozzles, for the main oxygen flow in the circuit (32) around a preferably water-cooled supply duct (16 ) are arranged for fuel and have a diameter d _a , which is larger than that by the formula

$${\text{d}}_{\text{a}}\text{= 1.13 (}\frac{\text{F}}{\text{η}}{\text{)}}^{\text{½}}$$

is a certain value, where d _{a is} measured in mm, F in mm² / MW and n is the number of nozzles; - A device (13, 38) is provided for preheating the main oxygen stream, which heats the main oxygen stream to at least 100 ° C; - The supply duct (16) for fuel at the outlet (19) generates a pulse current of at least 0.5 N / MW based on the combustion power; - The feed channel (14) for the primary oxygen flow on the central axis (10) of the feed channel (16) for fuel is arranged and at the outlet (19) has a cross section with an oxidation ratio of <0.01, preferably 0.004 of the stoichiometrically required amount of oxygen generates a speed of at least 1.5 m / s, the primary oxygen stream at the outlet (19) partially oxidizing with the emerging fuel and then mixing with the main oxygen stream and recirculated combustion exhaust gas. Burner according to claim 11,
characterized in that
the water-cooled feed channel (16) for fuel at the water-cooled outlet (19) is at a distance A from the nozzles (24 to 31) for the main oxygen flow, A being from the outside diameter of the feed channel (16) for fuel to the outside diameter of the nozzles (24 to 31) is measured at the outlet (19) and has a value which is less than 3.9 d _a . Burner according to claim 11 or 12,
characterized in that
the minimum center distance L of adjacent nozzles (24 to 31) for the main oxygen flow is a multiple, preferably at least 3 times the nozzle diameter d _a . Burner according to one of Claims 11 to 13,
characterized,
that the outlet channels (11, 12) of the nozzles (24 to 31) for the main oxygen flow are arranged parallel to the outlet channel (9) of the water-cooled feed channel (16) for fuel. Burner according to one of claims 11 to 14,
characterized,
that the feed channel (14) for the primary oxygen flow in the water-cooled feed channel (16) for fuel is designed to be displaceable and the exit of the feed channel (14) for the primary oxygen flow, preferably ends with the outlet of the water-cooled feed channel (16) for fuel. Burner according to one of claims 11 to 15,
characterized,
that the water-cooled feed channel (16) is designed as a nozzle that narrows to the outlet of the fuel and constricts the fuel flow. Burner according to one of claims 11 to 16,
characterized,
that the supply channel (14) for the primary oxygen flow with a sensor (34), preferably a UV light receiver, for detecting flame conditions is connected, which is connected to an evaluation unit (8) for flame monitoring.

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